Two component polyurethane systems find broad application as paints, foams, fibers, and nonporous moldings. Where the crosslinking partners are reactive even at room temperature, the polymer is obtained by mixing and subsequently reacting an isocyanate group-containing component with a hydroxyl group-containing component. The reaction here of the isocyanate group with the hydroxyl group is frequently catalyzed with addition of basic compounds, such as tertiary amines or compounds containing amidine groups. Examples of such compounds are 1,4-diazabicyclo[2.2.2]octane or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). A markedly higher catalytic activity, however, is possessed by organometallic compounds, such as dibutyltin dilaurate or various zinc carboxylates. The amount of catalyst used is determined on the one hand so as to ensure a sufficiently long processing life for the area of application, and on the other hand so as to attain, as early as possible following application, a profile of properties that is sufficient for subsequent operations. For automotive refinish in particular it is important to attain a rapid assembly strength.
The tin-based catalysts well-known from the prior art, such as dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, dibutyltin maleate or tetrabutylstannoxane diacetate, are effective catalysts for the formation of the urethane bond, but these compounds are toxic. On account of the toxicity of many tin compounds, attempts have been made for a long time already to find substitute catalysts suitable for coating material compositions.
Accordingly, the article “Catalysis of the Isocyanate-Hydroxyl Reaction by Non-Tin Catalysts” by Werner J. Blank, Z. A. He, and Ed. T. Hessell from King Industries Inc., describes alternatives to the conventional tin-containing catalysts, on the basis of various metal salts and metal complexes, such as zirconium chelates, aluminum chelate, and bismuth carboxylate.
WO 2009/135600 describes catalysts based on N-heterocyclic carbenes for the synthesis of polyurethanes. EP 1 460 094 describes catalysts based on metal salts in conjunction with bicyclic tertiary amine compounds and a quaternary ammonium salt. U.S. Pat. No. 4,006,124 describes general amidine-metal complexes as catalysts for the isocyanate polyaddition reaction. WO 2004/029121 discloses the use of acids having a pKa of between 2.8 and 4.5 as catalysts for the urethane reaction. A failing common to all these catalysts is that they result either in excessively slow curing of the coating system or else in a curtailed working time (pot life). Moreover, a number of the catalysts identified above cause discoloration of the coating materials or a subsequent yellowing of the applied coatings. Compounds of this kind are unsuitable for high-grade clearcoat systems. In addition, the publications identified above also describe cadmium and lead-containing catalysts, which on account of their toxicity are also out of contention as alternatives to tin-based catalysts. The bismuth- or zirconium-based catalysts described in the publications identified above are sensitive to hydrolysis and ought therefore only to be diluted in the curing agent. On account of the sensitivity to hydrolysis, coating systems based on bismuth and zirconium catalysts, moreover, possess a short shelf life.
As already described above, amines may also have a catalyzing activity on the polyurethane reaction. Nevertheless, their pot life is insufficient for processing in the automotive refinish segment. On account of the high vapor pressure of amines of low molecular mass, the only amines, if any, that are contemplated for this application are those such as diazabicycloundecene (DBU) or diazabicyclononene (DBN). Use of these amines, though, leads to strongly colored mixtures, and the resultant catalyzed coating systems have a propensity to unacceptable yellowing.
US 2006/0036007 discloses organometallic complexes as catalysts for the crosslinking of polyurethane-based systems. Described therein in particular are amidine compounds of zinc for the catalysis of the reaction of hydroxyl group-containing components with isocyanate group-containing components. These catalysts are said to be adsorbed on fumed silica. In US 2006/0247341 and
US 2009/0011124, amidine-zinc complexes are likewise used as catalysts in 2-component polyurethane coating systems. These catalysts are said to be particularly stable to hydrolysis and to be suitable also for use in aqueous systems. Nevertheless, the catalyst systems disclosed in these three documents do not exhibit sufficient curing properties. A comparison of the reactivity of these systems with tin-catalyzed systems shows that these zinc-amidine-based catalysts have poorer curing properties for a given processing life.
The object on which the invention is based is that of providing coating systems with suitable catalysts which exhibit not only rapid curing of the system but also a long processing life. Moreover, the coating systems are to confer on the user the possibility of rapid further-processing of the surfaces/articles coated with the systems. These systems, furthermore, are not to exhibit any changes in color before or after curing. In the area of clearcoats in the automobile industry, in particular, the requirements imposed on the inherent color of the systems are exacting. Thus the catalyst must not have an inherent color, and nor must it, together with the customary coating components, lead to any discoloration when the coating material is mixed or cured. The catalyst, moreover, ought to be able to be added to the coating system from the outset. Admixing the catalyst to the coating systems from the start, however, is not to have any adverse effect on the shelf life of the coating composition. Additionally, the catalyst ought to be insensitive to hydrolysis, since, even in systems in organic solution, the typically high concentration of hydroxyl groups can lead to a reduction in catalyst activity over the storage life. In the automotive refinish segment especially, an extremely long shelf life even at relatively high temperatures is an advantage.